Some implantable sensors are highly sensitive to motion artifacts, which are otherwise referred to herein as motion induced noise. An example of such a sensor is a photoplethysmography (PPG) sensor. A PPG sensor relies on the transmission and reception of light signals to produce a PPG signal that can be used to monitor various parameters, such as, but not limited to, heart rate, respiration rate, arterial pressure, and arterial oxygen saturation levels. A PPG signal can also be used for adjusting pacing parameters such as AV delay and VV delay for pacing optimization. Accordingly, it may be useful to implant a PPG sensor within a patient, so that specific parameters can be chronically monitored or adjusted based on a PPG signal. However, it would be undesirable and potentially detrimental to use such a sensor signal when it is corrupted by motion induced noise. For example, where a PPG signal is being used to adjust a pacing parameter, it may be detrimental to use the PPG signal to adjust the pacing parameter when the PPG is corrupted by motion induced noise.
One way to detect when motion is present is to use a separate motion sensor, such as an accelerometer. However, including a motion sensor can add to the cost and complexity of an implantable system. Further, even if an implantable system already includes a motion sensor, it would add to the complexity of the system to require that the output of the motion sensor be used to make decisions with respect to another sensor that is not specifically intended to detect motion but which is nevertheless sensitive to motion induced noise. Accordingly, it would be useful to provide new ways for determining levels of motion induced noise in signals produced by sensors (e.g., PPG sensors) that are sensitive to such noise.